In wireless communications, in a scenario of using a low-frequency carrier, a wireless signal has a relatively small path loss, and a beam formed at each antenna port is a wide beam; and therefore, users of an entire cell can be covered. For example, as shown in FIG. 1, a broadcast channel, a system message, paging, and the like of a network device may be transmitted by using a wide beam, to better cover a terminal device 1 and a terminal device 2.
However, in a high-frequency scenario, as shown in FIG. 2, a path loss of a wireless signal increases. If a wide beam is still used for transmission, cell coverage becomes very small and cannot cover a relatively remote terminal device 3 and terminal device 4.
An antenna distance may become smaller in high frequency, so that a unit area can accommodate more antennas. Therefore, a large antenna gain may be obtained by using a massive multiple-input multiple-output (massive MIMO) beamforming technology, to compensate for the path loss. There may be a large quantity of antennas even hundreds of antennas in massive MIMO. When a large antenna gain is obtained, a formed beam is very narrow in width. One narrow beam can cover only a part of an area, but cannot cover all users in a cell. For example, as shown in FIG. 3, a beam B2 can cover only a terminal device 5, but cannot cover a terminal device 6.
In a high-frequency multi-beam transmission scenario, to serve the users in the cell, different beams may need to serve the users in the cell in a time division manner, and the beams need common channels such as a broadcast channel, a synchronization channel, and a control channel to cover all the users in the cell, so that the users in the cell are synchronized and obtain a necessary system message to access the cell.
A beamformed control channel is sent by using an existing method for sending a control channel. For example, an existing control channel is a physical downlink control channel (PDCCH), and a beamformed control channel is a beamformed PDCCH. In this case, in one subframe, there may be both the PDCCH with large coverage used to serve an old terminal device, and the beamformed PDCCH used to serve a terminal device in enlarged coverage. The PDCCH with wide coverage is demodulated by using a common reference signal (CRS) with wide coverage. However, the beamformed PDCCH cannot be demodulated by using the existing CRS, and needs to be demodulated by using a beamformed reference signal. Therefore, when the beamformed PDCCH is used, a UE cannot correctly receive the beamformed PDCCH if there is no corresponding beamformed reference signal.
Therefore, how to transmit a beamformed control channel and a reference signal of the beamformed control channel becomes a problem that urgently needs to be resolved.